|Solar coronal holes
An area in the solar corona which appears dark in X-rays and ultraviolet light. The gas density in these areas are very low, about 100 times less than that of coronal active regions. The magnetic field lines in a coronal hole extend out into interplanetary space rather than returning to the Sun's surface, as they do in other parts of the Sun (open magnetic field line). Ionized hot gas can escape easily along such a path, and this brings about high speed solar wind streams.